1. Field of the Invention
The present invention relates to a linear drive device utilizing a linear motor, and particularly a linear drive device utilizing a linear motor which is provided with an encoder for operation control.
2. Description of the Related Art
In various fields relating to office automation equipment such as copying machines, image scanners and printers, to factory automation equipment such as X-Y tables and object transporting apparatuses, and optical equipment such as cameras, linear drive devices utilizing linear motors have been utilized for linearly moving objects or members.
A linear drive device of a so-called moving coil type is well known as a kind of drive device utilizing a linear motor. This type of linear drive device has a stator having a field magnet provided with N- and S-type magnetic poles arranged alternately to each other, and extending in a given direction, and a movable piece having an armature coil opposed to the field magnet, and being reciprocatively movable along the stator.
In the drive device of this type, coil components forming the armature coil are supplied with currents depending on the polarities of the magnetic poles of the field magnet opposed to the coil components, respectively, so that a drive force for driving the movable piece in an intended direction can be produced by an interaction between these currents and the magnetic field produced by the field magnet.
A sensor for sensing change in magnetic poles of the field magnet is mounted on a movable piece for controlling power supply to the armature coil so as to produce the movable piece drive force.
Such a sensor for the field magnet is generally formed of a magnetoelectric device such as a Hall element or a magnetoresistive element (e.g., MR element) which can issue an electric signal depending on the polarity of the magnetic pole and/or an intensity of the magnetic field.
The linear drive device utilizing the linear motor usually employs an encoder (usually, linear encoder) for sensing and controlling positions, speeds or the like of the movable piece, or an object, member or the like which is connected to and driven by the movable piece.
The linear encoder can be roughly classified into a magnetic encoder and an optical encoder.
The magnetic encoder is formed of a magnetic encoder scale in which N- and S-type magnetic poles are arranged alternately to each other in the movable piece moving direction with a pitch smaller than the pitch of the magnetic poles in the field magnet, and a magnetic sensor for reading magnetic information on the scale. The magnetic sensor usually employs a magnetoelectric element such as a magnetic resistance element (MR element) or a Hall element which issues an electric signal depending on the polarity of the magnetic pole of the magnetic encoder scale and/or the intensity of the magnetic field.
The optical encoder is formed of an optical encoder scale which is provided with two kinds of optically different surfaces arranged alternately to each other in the moving direction of the movable piece, and an optical sensor reading optical information on the scale. The photosensor may employ a photoelectric conversion element such as a photodiode or a phototransistor which can issue an electric signal depending on a quantity of light coming from the optical scale, or a one-packaged optical sensor, i.e., a combination of a light emitting diode (LED) emitting light beams to an encoder scale and a photoelectric conversion element.
If the magnetic encoder is to be employed in fields requiring driving of objects or the like under precise control of positions and/or speeds and, more specifically, in the field, e.g., of the office automation equipment, the detection of the scale information by the magnetic sensor must be able to suppress error, and the detected information must have a high precision.
However, in the structure employing the magnetic encoder with the magnetic encoder scale formed at the stator together with the field magnet, a magnetic force of the magnetic encoder scale is usually smaller than the magnetic force of the field magnet, and the magnetic encoder scale is susceptible to magnetic interference with the field magnet, and the sensor reading the magnetic information on the encoder scale is susceptible to the magnetism of the field magnet, so that the accuracy of sensing the magnetic information on the encoder scale by the sensor may be lowered, and an error in sensing may occur. If an error in sensing of the encoder scale information occurs, the linear drive device may not operate smoothly with a high accuracy, or a malfunction may occur.
In view of the above, the following has been proposed, e.g., for the linear drive device of such a type that a field magnet and a magnetic encoder scale are arranged on a stator, and a magnetic sensor reading magnetic information on the encoder scale is mounted on a movable piece which can move along the stator.
According to one of the proposals, the field magnet and the magnetic encoder scale are spaced from each other so that an offset variation in output waveform of the magnetic sensor for the encoder, caused by an influence by the field magnet, is kept within a predetermined range. It is also proposed that a magnetic shield wall is arranged between the field magnet and the magnetic encoder scale.
According to the former, however, the magnetic flux distribution of the field magnet must be narrow so as to keep the offset variation in output waveform of the encoder magnetic sensor within the predetermined range. This lowers the force for driving the movable piece, and may impede stable and smooth operation of the movable piece.
In the latter, although the magnetic shield wall is arranged between the field magnet and the magnetic encoder scale, the magnetic force produced from the field magnet partially passes over the magnetic shield wall toward the scale, resulting in disadvantages such as deterioration of the sensing accuracy of the magnetic sensor.
The following can also be pointed out in connection with the linear drive device utilizing the linear motor.
The sensor sensing the change in magnetic polarity of the field magnet on the stator as well as the sensor reading the encoder scale information may be deteriorated to lower the sensing accuracies when a thermal influence is exerted thereon. In the prior art, however, the movable piece is arranged and used without taking the heat generation of the armature coil into consideration.
For example, the Hall element, which is a kind of magnetoelectric element and can be employed as the field magnet sensor or the magnetic encoder sensor, may be typically selected from Hall elements containing InSb (indium antimony), InAs (indium arsenic) and GaAs (gallium arsenic). Outputs of these elements vary, to one degree or another, depending on a surrounding temperature. Particularly, the InSb-contained Hall element has bad temperature characteristics although it produces a large output signal (Hall voltage), and the output voltage thereof varies to a large extent depending on the temperature. The MR element, which is a kind of magnetoelectric element, has such characteristics that its output lowers with a rising of temperature.
The optical sensor which is employed in the optical encoder already described has likewise such disadvantage that it may deteriorate and the sensing accuracy may lower due to a thermal influence.
As described above, the sensors for the field magnet and encoder may deteriorate due to heat, and outputs thereof may vary, in which case information to be sensed cannot be sensed accurately, resulting in such disadvantages, for example, that the linear drive device may not operate accurately and smoothly, and may malfunction.
In spite of the above facts, the drive devices utilizing the linear motors in the prior art are provided with the sensors which are arranged without taking the influence by heat generation of the armature coil into consideration. Although one of the sensors for the field magnet and the encoder may be arranged at a position (e.g., under the stator) less affected by the thermal influence of the armature coil, the other sensor is arranged at a position (e.g., above the stator) which is susceptible to the thermal influence. In the structure where one of the sensors is arranged at a position susceptible to the influence by the heat of the armature coil, and thereby cannot achieve an intended information sensing accuracy, such disadvantages occur that the linear drive device cannot operate accurately and smoothly as a whole, and a malfunction occurs. These disadvantages are liable to occur among the drive devices utilizing the linear motor of the moving coil type with the armature coil of the movable piece fitted around the stator and, in other words, are liable to occur in the drive device utilizing the linear motor of a so-called shaft type.
The following can also be pointed out in connection with the linear drive device of the moving coil type, and particularly the linear drive device utilizing the linear motor of the shaft type.
When the movable piece driving a driven object is connected to the driven object and particularly one of its opposite ends which are spaced in a direction crossing the drive direction of the driven object, i.e., a predetermined direction in which the driven object is to be driven linearly, a so-called yawing is liable to occur, and more specifically the movable piece is liable to swing around an axis which is perpendicular to both the moving direction of the movable piece and the width direction of the driven object crossing the moving direction. If this yawing occurs, the positional relationship between the field magnet sensor (i.e., sensor for the field magnet) mounted on the movable piece and the field magnet may deviate or become instable, and/or the positional relationship between the encoder sensor (i.e., sensor for the encoder) and the encoder scale may deviate or become instable. This prevents accurate and stable operation of the linear drive device, and causes problems such as a malfunction.